The coherent detection of optical communication signals using the phase shift keying (PSK) modulation format has long been known to offer superior performance relative to direct detection techniques, using conventional direct detection or differential detection modulation formats. First, PSK coherent detection allows higher signal-to-noise ratios (SNRs) to be achieved relative to the direct detection techniques. Second, by preserving and mapping the optical phase of the PSK signal to the electrical domain, PSK coherent detection enables efficient chromatic dispersion (CD) and polarization mode dispersion (PMD) compensation in high speed optical communication systems using low speed adaptive digital filtering. However, building a high performance coherent receiver at reasonable complexity and cost still remains a challenge. One of the critical aspects associated with a coherent receiver is its ability to precisely estimate the relative optical phase and state of polarization between the incoming data signal and the Local Oscillator (LO), and its ability to lock the LO's phase and polarization to the data signal's phase and polarization.
The current state of the art for estimating/locking phase and polarization includes four different techniques. The first technique relies on building phase locked loop (PLL) and polarization tracking functions directly in the analog domain, detecting the phase and polarization difference between the LO laser and the incoming data signal, and stabilizing the LO phase and polarization accordingly. The second technique utilizes an un-stabilized LO and operates in the digital domain by using high speed, real time analog-to-digital conversion (ADC) and digital signal processing (DSP) technology to extract the estimated optical phase and state of polarization in the electrical domain. The third technique also utilizes an un-stabilized LO, and analog signal processing (ASP) in the radio frequency (RF) domain to extract the phase and polarization. The fourth technique utilizes the transmission of a data signal along with a LO pilot tone that is amplified at the receiver.
A major difficulty associated with deploying a coherent detection system is the PLL that locks the phase of the LO field to the phase of the signal carrier field of the optical communication signal. It is desirable to have a noiseless PLL, with zero acquisition time. However, in reality, a compromise must always be struck between the noise bandwidth and the acquisition speed. This contributes to signal degradation on one hand and limits the phase acquisition and tracking speed on the other hand. In addition the current state of the art DSP based approach requires the use of a parallel architecture, as used in high speed ADCs, causing a feedback delay. This necessitates the use of narrow linewidth transmission and local oscillator lasers with sufficiently long coherence times. Therefore, the use of distributed feedback lasers with a linewidth of ˜1 MHz becomes problematic in DSP based coherent detection, especially for modulation formats with a dense constellation. Another constraint associated with DSP based coherent detection is the limitation on communication speed due to the fastest ADC and DSP devices that are available. For example, in digital systems, the sampling rate is typically 2× the symbol rate, such that, for symbol rates approaching 40 Gsymbol/s or higher, the required ADC and DSP devices are not yet available. The same problem related to narrow linewidth transmission and local oscillator lasers exists for other coherent detection systems that utilize either analog PLLs directly in the optical domain or phase recovery in the electrical RF domain.
The transmission of a data signal along with an LO pilot tone solves the problem of maintaining a constant phase and frequency relationship between the data signal and the LO, as both originate from the same optical source and traverse the same path. One possibility is to add the LO pilot tone at the transmitter on a polarization that is orthogonal to the data signal. However, in order to achieve practical coherent detection, the LO pilot tone and the data signal must be separated at the receiver, and the LO pilot tone must be amplified. The LO and the data signal are typically separated via one of two mechanisms.
In the first mechanism, the LO and the data signal are separated at the receiver via polarization tracking, and the LO is preferentially amplified. Note that this mechanism is applicable only to systems with an LO pilot tone polarization that is orthogonal to the data signal. Thus, this mechanism precludes the use of polarization multiplexing for signal capacity doubling. Additionally, PMD and polarization dependent gain/loss reduce polarization orthogonality, in general.
In the second mechanism, the LO pilot tone is added to the data signal such that it is sufficiently separated from the data signal spectrum in frequency (i.e. wavelength). As a result, the LO pilot tone can be optically filtered out at the receiver and preferentially amplified. The requirement for such frequency separation, set by the ability of the optical filter(s) to provide selective filtering, reduces the spectral efficiency of WDM systems, however. Furthermore, it requires receiver electronics to operate at a much higher bandwidth, so as to keep the mixing between the LO and the data signal spectra within the receiver electrical bandwidth. In addition, due to the spectral separation of the LO pilot tone and the data signal, the phase of the LO pilot tone relative to the phase of the data signal depends on the CD. As a consequence, the LO-data signal phase difference can vary over time due to fiber temperature induced dispersion variations, etc. Although such relative phase variations are slow compared to the relative phase variations for the external LO scheme, they nevertheless must be tracked by an additional PLL.
A novel technique has been proposed for the coherent detection of optical signals with Brillouin amplification of the associated signal carrier (see Vladimir S. Grigoryan and Michael Y. Frankel, “Systems and Methods for the Coherent Non-Differential Detection of Optical Communication Signals,” U.S. patent application Ser. No. 11/875,622, filed Oct. 19, 2007). The major advantage of this technique relative to conventional coherent detection techniques is that it does not require a fast PLL. However, it still requires polarization tracking to align the incoming data signal polarization with one of the axes (i.e. either fast or slow) of the polarization maintaining fiber used, As the propagation distance of an optical communication signal in a fiber network increases, the speed of evolution of the polarization state of the optical communication signal also increases, such that it can sometimes exceed 10's of kiloradians/s for a long haul transmission. Existing polarization tracking devices are not capable of polarization tracking at such high speeds. Thus, the lack of a high speed polarization tracking device becomes a barrier for use of this Brillouin amplification coherent detection technique in long haul fiber networks.
With the continued growth of data traffic over optical networks worldwide, the deployment of new systems with higher capacity per single wavelength is becoming increasingly important. In particular, a 100 Gb/s Ethernet solution is widely considered to be the platform for the next generation of optical networks. Given the severe constraints associated with 100 GHz electronics, optical modulators, and photodiodes on the one hand and high bandwidth optical communication signal impairments due to fiber dispersion and nonlinearity on the other hand, in order for 100 Gb/s Ethernet to become a practical solution, advanced modulation formats are required to reduce the optical bandwidth and symbol rate. Polarization division multiplexing (PDM) is a practical technique used to double channel capacity without increasing channel bandwidth and symbol rate. PDM combined with the quadrature phase shift keying (QPSK) modulation format is widely considered to be one of the most practical solutions for 100 Gb/s Ethernet systems as it allows for the reduction of the symbol rate to 28 Gbaud/s (25 Gbaud/s for the payload, and an additional 3 Gbaud/s for the framing overhead). However, an inherent drawback of PDM is that it requires a polarization tracking device at the receiver to align the incoming signal polarization to a polarization beam splitter to demultiplex the signal. The state of polarization of an optical communication signal traversed through a fiber undergoes random evolutions over time because of the unpredictable drift of the fiber polarization axes.
There are three techniques used in state of the art polarization tracking devices. The first technique, analog polarization tracking, uses a polarization controller based on a sequence of several electro-optical crystal wave plates with processor controllable voltages applied to them, resulting in controllable retardation and orientation angles in each of the wave plates. The associated polarization tracking error is up to 0.13 radians on the Poincaré sphere with the tracking speed not faster than several kiloradians/s. The second technique is based on coherent detection. It uses a polarization diversity receiver, an unstabilized LO, and operates in the digital domain by means of a high speed real time ADC and DSP technology to extract the estimated state of polarization and phase in the electrical domain. The third technique also uses a polarization diversity receiver and unstabilized LO, but implements an analog processing in the RI domain to extract the polarization and phase.
As the propagation distance of an optical communication signal in a fiber network increases, the speed of evolution of the signal state of polarization increases such that it can, sometimes, exceed 10's of kiloradians/s for a long haul transmission. In addition, the speed of evolution of the signal state of polarization can exceed 10's of kiloradians/s even if the drift of the polarization axes in the fiber is slow but distributed high speed polarization scramblers are used. Such distributed high speed polarization scrambling is widely considered to be an efficient and the most economic technique for mitigating PMD. Existing analog polarization tracking devices are not capable of polarization tracking at such speeds. In addition, the 0.13 radians of polarization tracking error in state of the art analog polarization tracking devices can result in significant penalties due to the crosstalk of the polarization channels. State of the art DSP based polarization tracking techniques rely on coherent detection schemes with an external LO. Again, a major difficulty associated with deploying a coherent detection system is the PLL that locks the phase of the LO field to the phase of the signal carrier field of the optical communication signal. It is desirable to have a noiseless PLL, with zero acquisition time. However, in reality, a compromise must always be struck between the noise bandwidth and the acquisition speed. This contributes to signal degradation on one hand and limits the phase acquisition and tracking speed on the other hand. In addition the current state of the art DSP based approach requires the use of a parallel architecture, as used in high speed ADCs, causing a feedback delay. This necessitates the use of narrow linewidth transmission and local oscillator lasers with sufficiently long coherence times. Therefore, the use of distributed feedback lasers with a linewidth of ˜1 MHz becomes problematic in DSP based coherent detection. Another constraint associated with DSP based coherent detection is the limitation on communication speed due to the fastest ADC and DSP devices that are available. For example, in digital systems, the sampling rate is typically 2× the symbol rate, such that, for symbol rates approaching 40 Gsymbol/s or higher, the required ADC and DSP devices are not yet available.
Thus, high speed and accuracy analog polarization tracking becomes a barrier in using a PDM technique in a modern fiber communication system. One consequence is the inability to use distributed polarization scrambling technology for PMD mitigation in PDM systems, which results in higher vulnerability and lower tolerance of the PDM systems as it relates to PMD.